The Optoelectronics Group at Strathclyde has been involved in research into fibre optic sensors and related areas since its inception. Its current activities include at the sensor level, optically activated mechanical resonators, interferometric and polarimetric sensors for composite material testing, fibre optic gyroscopes and surface plasmon resonance immunosensing. These activities are complemented by a programme on single mode optical fibre and integrated optic components embracing modulators, polarizers, directional switches in all-fibre, glass integrated optics and silicon based integrated optics. The sensor interests are taken into the system domain via research into passive fibre optic sensor networks and the closely related topic of signal processing systems based upon optical guided wave concepts. The group's endeavours range from basic research through to technology transfer and design consultancy in the entire spectrum of the technical themes outlined here.

The research program at Stanford University is centered around fiber-optic devices that perform specific functions. The devices of our interest range from simple passive and active components to relatively complex sensor systems and optical signal processing devices. Many devices introduced by our research program, that are originally intended for use mainly in fiber sensors, are finding increasing applications in optical telecommunications as the overlap between the two major fiber optics fields has increased substantially.

Fiber optics provide a technology which may be readily applied to chemical sensing. This application is the main thrust of fiber optic sensor development at the University of Washington. Researchers from a number of academic departments are investigating fiber optic based chemical sensors for applications in industrial, environmental, and biomedical monitoring. A brief overview is given with several examples of current sensor projects.

The Fiber Optic Materials Research Program (FOMRP) was established in July 1985 at Rutgers, the State University of New Jersey through major funding from the New Jersey Commission on Science and Technology. This summary outlines some of the important features of FOMRP including basic program goals and objectives, current research initiatives and thrust areas, the role of corporate and government sponsors, present and future availability of research facilities, of recent scientific publications, the make-up of FOMRP faculty and graduate students and a listing of recent scientific publications.

Research in the Laboratory for Lightwave Technology is focused on novel applications of specially designed optical fibers. Emphasis is placed on new techniques of dopant incorporation, using both MCVD (Modified Chemical Vapor Deposition) and OVD (Outside Vapor Deposition) to fabricate fibers that are primarily for non-telecommunications applications. The topics pursued are fibers for fiber lasers, nonlinear effects in fibers, in particular, second harmonic generation, fiber devices for the measurement of electric field, new techniques for the fabrication of bulk-glass gradient index lenses, fiber designs for embedded fibers for sensor elements of future "smart skin" composites, the poling of glasses to induce anisotropy or possibly optical activity, and finally, fiber designs for N x N fused taper couplers.

In this paper is presented the research work in Fiber Optic Sensors currently being developed at the Departamento de Engenharia de Telecomunicaaes of Universidade Federal Fluminense(TET-UFF) jointly with the Centro de Pesquisas de Energia Eletri-ca(CEPEL). It is given particular emphasis to the present state of development of the temperature, vibration and electrical current sensors.

The academic and industrial community active in the field of optoelectronics in this country is organizing now every two years a major international meeting on Optical Fibres and their Applications. The first meeting was held in 1976 and the fifth in the series during February 1989. The next meeting is scheduled for February 1991. It used to gather 150 contributed and invited papers and a few hundreds of participants both domestic and international. The proceedings of this meeting are now issued by SPIE-The International Society for Optical Engineering (vols. no 670 and 1085).

Common-mode perturbations are an important subset of the environmental variations which affect the accuracy and stability of interferometric fiberoptic sensors. These effects, among which are source wavelength drift and spatially uniform temperature and pressure variations, alter the phase shifts in each of the optical paths in the fiber circuit in proportion to the optical path length. Earlier work showed that a recirculating delay line fiberoptic sensor could be modified to compensate for common-mode perturbations, but at the cost of some loss in measurand sensitivity. In this paper, we employ transmittance-dependent electrooptic feedback in an enhanced Mach-Zehnder fiberoptic interferometer to compensate for common-mode perturbations, while enhancing by aproximately an order of magnitude the measurand sensitivity found in the conventional Mach-Zehnder fiber interferometer.

A detection scheme of the optical signal provided by an interferometric sensor using spectral modulation encoding techniques is presented. The device includes a broadband light source, an interferometric sensor and a receiver interferometer, both connected by multimode fiber. The optical signal going through the system is then analyzed with a low resolution spectrophotometer.

We show that high frequency phase-noise is frequency translated to the baseband in interferometric sensors using phase-generated carrier (PGC) demodulation. Under normal circumstances, this excess noise does not contribute significantly to the true low frequency phase-noise arising due to 1/f laser frequency jitter. However, when a phase noise stabilized laser is used, degradation in phase sensitivity of an interferometer can occur using PGC demodulation when compared with direct homodyne detection. The origins and nature of this noise are discussed, and experimental verification of the effect presented.

There are an increasing number of applications in short haul optical fiber communications and multi-sensor networks for which a very high spatial resolution (on a few cm scale) Optical Tine-Domain Ref lectometer (OTDR) is needed. For many applications, it is sufficient to be able to detect back-reflections from specific elements, such as connectors and optical switches, within the fiber link. However, in certain cases, such as for distributed sensing using optical fibers embedded within various mechanical structures, it is important to be able to measure the Rayleigh backscattering from the fiber. In order to achieve centimeter scale spatial resolution along a fiber, it is necessary to employ optical sources and detectors with response on a 100ps timescale. In this paper, we describe the results of an investigation into the limiting capability in terms of spatial resolution and return signal strength which can be achieved using a photon counting method of detecting the backscatter signals from the fiber system under test. The bulk of the paper is concerned with system operation at a wavelength of 0.85 μm; some preliminary results obtained at 1.3 μm are also described.

This paper describes a new method that we have developed to generate Bragg grating filters in germania-doped communication fibers and discusses their applications as sensors. The gratings are formed by exposing a short section of the core, through the side of the fiber, to an interference pattern of intersecting coherent beams of UV light. A permanent periodic index modulation is produced by the interference fringes. It forms a phase grating which acts as a band rejection filter, passing wavelengths that are not in resonance with the grating and strongly reflecting wavelengths which satisfy the Bragg condition. The grating wavelength is sensitive to changes temperature and strain. Measurements of the shift in Bragg wavelength with these quantities are reported and compared with computed estimates of the sensitivity. A large number of independent gratings can be easily written along a length of fiber to make a distributed sensor system. We have also demonstrated that Bragg gratings can wavelength tune laser diodes, which could be used for detecting the wavelength shift of the Bragg sensors.

An optical source, whose intensity is modulated by a band-limited analogue noise waveform, is used to generate transverse vibrations in a micromechanical silicon resonator. The vibrations are generated by a photothermal process in which optical energy is absorbed at the surface of the resonator thereby producing a thermal wave whose a.c. component causes a periodic expansion of the resonator. A pressure sensor incorporating a micromechanical resonator sensor has been fabricated, and the pressure sensitivity of this device, when driven by the band limited noise source, is reported.

The use of frequency ramped (chirped) laser diode sources has increased dramatically through their use in interferometric heterodyne systems, fiber fault location systems, and fiber sensor multiplexing. Frequency chirping is typically achieved via either direct modulation of the device, modulation of the diode's bulk heatsink, or varying the diode's bias current with both methods having advantages and disadvantages. In this paper we present a direct comparison of these methods' chirping performance (sweep linearity and range) on various commercially available laser diodes. Measurements were performed using a high resolution spectrometer system capable of realtime monitoring of the temporal evolution of the spectral response of a laser diode's output. Three dimensional light-current curves were determined and mode instability was investigated. Analysis of the resultant information was used to make some inferences regarding laser diode selection.

A fiber force sensor with heterodyne polarimetric detection has been realized. Stability, repeatability, dynamical behaviour and linearity have been tested. Data analysis, developed also with the Poincare' sphere representation, has given satisfactory results.

Recent research conducted in the Fiber-Optic Sensor Systems Laboratory at Florida Institute of Technology in fiber-optic strain sensors for use in smart structures has concentrated primarily on one-fiber interferometers. Sensors using this technique are of particular interest because they are rugged and provide reasonable strain sensitivities, typically a thousand times that of microbend sensors, while requiring only a single optical fiber. As in two-fiber interferometers, some method must be used to convert the sensor signal output to an absolute strain value. One technique is to provide active phase tracking to keep the sensor signal output at phase quadrature for maximum sensitivity and to eliminate fringe count uncertainty. This paper contains a description of a technique for active phase tracking in polarimetric sensors composed of high- and low-birefringence fiber using an electro-optic polarization modulator. On-going development of artificial neural processors for processing fiber-optic sensor signals from both polarimetric and few mode sensors, as well as generating control signals for actuators in smart structures, is also discussed.

After one year of the concept phase, a safety system for, among other applications, offshore oil rigs has been projected for a trial in the offshore application of fiber optic sensors. The technical advantages of fiber optics in offshore environment and in process plants are insensitivity to EMT, intrinsic safety and - if in future electrically passive, optically multiplexable fiber optic sensors (FOS) also become available - a reduction of cable cross section and weight. In the test and installation phase, which began in the middle of 1988, sensors were firstly ordered or further developed from existing sensors or sensor components. Tests in the laboratory have then been carried out. The sensors which passed the tests have been or will be installed parallel to the existing monitoring and emergencv-and-shut-down systems in a field test on an offshore oil rig. The trial will provide us with many answers to questions about how optical fibers and sensors can be applied in difficult and adverse environments.

United Technologies Research Center (UTRC) has investigated the Frequency-Modulated-Carrier-Wave (FMCW) technique for fiber optic displacement sensors and fiber optic multiplexed networks. There is especial interest in multiplexing techniques such as frequency-division, wavelength-division, and time-division for aerospace and industrial sensing applications. UTRC has looked at a number of these techniques for multiplexing fiber-optic sensors for engine control applications. In particular, UTRC has concentrated on coherent techniques because of the inherent sensitivity and robustness of these methods. In previous work, the FMCW technique was investigated using different lasers in a discrete optic Michelson interfeyometer, in which laser frequency chirp and coherence properties were studied. In an interferometric system, direct modulation of the laser diode with a linear ramp generates an output beat frequency. This beat frequency is proportional to the optical path length difference of the interferometer. Various schemes fo5 multiplexing fiber sensors using the FMCW technique have been suggested. The emphasis has been on increasing the sensor count through understanding the coherence and optical power requirements of the system; and minimizing the effect of cross terms by implementing various architectures and imposing restrictions on the information frequency band to be measured. This paper compares two approaches to the FMCW technique in which the effects of cross terms on the system performance is minimized by taking advantage of laser coherence properties and system design. A fiber-optic linear bus network consisting of three reflective sensors was constructed. The paper presents experimental results and analysis of the FMCW multiplexed system performance.

An intrinsic, wavelength-division-multiplexed, high- and low-sensitivity fiber-optic sensor is proposed and analyzed as a possible alternative to the existing optical fiber-sensing methods used in real-time temperature and strain evaluations. The sensor unites two existing technologies: (1) the dual core, single-mode, cross-talk phenomena exhibiting relatively low sensitivity and (2) the Fabry-Perot interferometric sensing phenomena that is known for its high sensitivity. The sensor uses the wavelength sensitivity of fused, tapered, single-mode couplers as a method of multiplexing the two widely exploited sensors outlined above. The sensor configuration consists of two independent coherent sources of differing wavelengths, a wavelength division multiplexer/demultiplexer, a center-cleaved, fused, biconnically tapered, single-mode coupler/splitter, and partially reflective, mirrored surfaces at designated locations along the fiber. Thus, a unique sensing arrangement having two independent (high- and low-sensitivity) sensors functioning simultaneously is realized.

A optical fiber flowmeter based on the correlation technique is developed.According to the small size of optical fiber probe and the advantage of correlation processing,the mathematical model established for the optical fiber correlation flow measuring system can be simplified reasonably by means of "frozen pattern"hypothesis,and a simple,convenient expression can be obtained. In the system,the signal gained from the transmitting probe is transferred through the optical fiber,and then processed by pre-amplifier,high speed recorder,finally,the signal is correlated by microprocessor ,and the velocity can be obtained from the delay time.

Normal multimode graded-index optical communication fibres have been operated at temperatures up to 1200°C with a view to determining their suitability for communication and sensing applications in very high temperature environments. To this end, scattering from fibres which have been repeatedly heated to temperatures up to 1200°C has been investigated with an OTDR operating at 850 nm. A complicated variation of scattering with temperature has been observed, particularly during the first heating of the fibre. However, with suitable pre-treatment the fibre scattering may be used for distributed temperature sensing at temperatures up to about 800°C. Above this temperature the fibre has limited life due to rapid crack propagation and breaking.

We propose a modified coherent rotation model to describe recently observed phenomena in the magnetostrictive responses of Metglas transducers used in fiber-optic magnetometers. A technique to extract the values of the transducer parameters is described. Predictions of the performance of a cylindrical transducer based on the measured values for the transducer parameters are presented and compared to the experimental result.

An optical fibre current sensor based on the Faraday effect has been designed and tested. The sensor, which is intended for use in high voltage power distribution systems, makes use of a single-mode fibre in the sensing region. A novel polarization analyser arrangement is included in the current sensor and the operation of this component is described. It is shown that a simple graphical representation of the state of polarization can be produced in real time. Preinstallation test results of the optical current sensor are given.

This paper describes the signal conditioning electronics for a very low frequency (VLF) fiber optic receiving antenna. Homodyne demodulation is performed using phase generated carrier techniques. The receiver electronics had a baseband signal of 10 kHz to 60 kHz and a carrier frequency of 404 kHz. The noise floor measurement of the high frequency demodulation electronics yielded 11 microradian/√qHz at 12 kHz. The design, development, and testing of the receiver will be discussed. Also, the system architecture and design concepts of the fiber-optic antenna are presented.

A Mach-Zehnder interferometer configured as a magnetic gradiometer common mode rejects the field. The nonlinear effect coupling the transducer in each arm of the interferometer results in a sensor output that contains signals proportional to both the field and the gradient if the sensor is slightly out of balance. This field signal can swamp the gradient unless care is taken to eliminate it. Analytically modelled data are presented showing this effect, and it's estimate of the "purity" of the gradient signal. To operate the gradiometer unambiguously, the field must be rejected by >60 dB.

The Faraday rotation is discriminated from linear birefringence in fiber current sensor by a time multiplexing of two different states of polarization at the input of the fiber. In this way an accurate current measurement can be carried out whatever the magnitude of the linear birefringence may be. A polarization multiplexing system using two identical laser diodes is presented. The experimental results and a method of compensating the imperfections of the optical components are also discussed.

A balancing procedure for a fiber optic magnetic gradiometer is presented. The gradiometer is a Mach-Zehnder interferometer with both arms configured as measurement arms. The procedure describes the nulling of DC environmental fields at each of the transducers without the difficulties associated with hysteresis when a previously nulled transducer is re-energized. The use of an additional AC signal at a frequency small compared with the dither is discussed, and balancing data are presented demonstrating the procedure.

When an oblate spheroidal shell having an aspect ratio aib > (2-v)112, where v Poisson's ratio, is subject to hydrostatic compression, the semi-major, a, and semi-minor, b, axes experience strain of opposite sign. If two optical fibers, which comprise the arms of an interferometer, are wound around the equatorial and meridional circumferences of the spheroid, pressure changes induce a differential optical phase shift in the interferometer. Calculations of circumferential strain, polar and equatorial displacement, lowest natural frequency, buckling pressure, and optical pressure sensitivity of such a sensor are presented for thin shells. Sample designs based on these calculations are compared to other fiber-optic hydrophones of similar dimensions and materials. Preliminary designs will be presented.

Pressure gradient, pressure, and accelerometer hydrophones have been constructed and tested. These hydrophones were based on single mode fiber optic directional couplers made by drawing and fusing. All couplers were fabricated out of commercial fiber optic cable using an electric furnace. The couplers were potted in several kinds of silicone rubber in a number of different sizes and configurations. Acoustically produced stress occurring near the coupler fusion joint causes the output of the coupler to change. These optical power level changes were remotely monitored by photodiodes. Gradient hydrophones, constructed by the described techniques, were shown to exhibit very high sensitivities, large linear dynamic range, and deep directional nulls with no observable hydrostatic pressure response. Pressure type hydrophones have a fairly flat response down to 10 Hertz with sensitivities better than -160 db rel 1V perp. Pa. By mass loading the silicone rubber membranes which pot the couplers, accelerometer-type hydrophones can be physically very small in size and have very low frequency resonances. This paper discusses the very inexpensive methods used to construct variable coupler hydrophones. A noise analysis shows that the sensor's projected self-noise is below sea state zero noise.

Two versions of a fast-response microbend pressure sensor have been developed for deployment in high temperature environments. Aluminum-coated fiber is used in a drum configuration in the sensors which employ a unique "fingered" flexural diaphragm technique to induce microbend attenuation. A dedicated test facility consisting of a shock tube and customized furnace has been established for high-temperature pressure sensor evaluation. The shock tube delivers reproducible, fast rise-time pressure pulses. Sensor response has been obtained using single mode fiber. The furnace, which has been built around the shock tube, and which maintains a heated environment to 750°C, has not yet been used with the fiber sensors.

The design and preliminary performance results of a fiber-optic (FO) Michelson interferometric hydrophone are described.

A prototype fiber optic pressure sensor has been demonstrated which is capable of accurate measurement of gas pressure at ambient temperatures up to 650 C. Based on the photoelastic effect, the pressure sensor incorporates a transparent diaphragm fabricated from fused silica. This choice of materials provides advantages of practical and inexpensive fabrication, complete absence of residual stress or natural birefringence, near-perfect elasticity, and resistance to temperatures up to 1100 C. The light used to address and interrogate the sensor diaphragm is transmitted on commonly-available aviation-type fiber optic cable, allowing the use of relatively high powers and the associated high signal-to-noise ratio. Unpolarized and incoherent light is used, so that any susceptibility to cable perturbations is minimized. This requires that the light be polarized within the sensor itself, which was achieved by the creation of a unique high-temperature polarizer fabri-cated from the same material as the sensor diaphragm. The necessary fiber optic connectors were located on remote standoffs, where the tem-perature could be kept below 150 C, and the optical power was conveyed further within the sensor by silica light pipes. Tests of the prototype sensor have demonstrated measurement of gas pressure up to 600 psi, at sensor temperatures from 25 to 650 C, with a total error at any temperature of 2 psi or less.

A new sensor has been developed for high hydrostatic pressure measurement based on displacement of the absorption edge in a semiconductor under the effect of pressure. The sensing element is composed of a GaAs single-crystal which has been attached to two multimode optical fibers. The crystal is placed inside a high-pressure chamber while fibers are used to carry the optical signal in and out. The present configuration allows determination of pressure up to 100 MPa. Temperature effects in the range 10°C to 50°C have also been investigated.

A high accuracy interferosetric fiber-optic pressure sensor is disclosed in this paper. The var-ying pressure-induced displacement is measured by a micro-interferometer installed in the sensing probe of this sensor. The adoption of a tong 0.6328gm single-mode fiber enables the coherent laser beam to be transmitted into the sensing probe and the varying interferosetric light beam to be out-transmitted to the opto-electronic receiver which say be located faraway from the sensing probe. This sensor is,therefore, suitable for the pressure measurements in hazardous environment where electromagnetic interference, poisonous gases, etc. are involved. The measuring accuracy of this sensor is 0.5% of the pressure changes being measured, The dynamic range varies from 3Pa to 125 KPa.

Sensitivity is one of the most important features for a sensor. The paper makes study on the photoelastic optic fiber pressure sensor. Based on the predecessor's work,the paper puts stress on the thin plate sensing element --- a new configuration of the photoelastic sensing element. The modulation theory of the thin plate sensing element is revised and perfected by a thorough theoretical analysis. The theory reveals that the adoption of thin plate sensing element can largely increase the sensitivity of the sensor by decreasing its thickness. In addition,an experimental prototype of photoelastic optic fiber pressure sensor with thin plate sensing element is provided. The experimental results coincide well with the theory, and relative error is less than 4% in total measuring range.

Fiberoptic inertial rotation sensors (or gyroscopes) so far are based on the Sagnac effect in either a multiturn interferometer1 or a resonator². Both these approaches constitute the fiberoptic implementation of previously demonstrated Sagnac interferometer3 gyroscopes and resonator4 gyroscopes employing bulkoptic components. However, there has not yet been a fiberoptic analog of the ring laser gyroscope5'6, which has always been a bulk-optic device. In this paper, we present preliminary data on a fiberoptic ring laser gyroscope using stimulated Brillouin7 scattering as the laser gain medium. To our knowledge, this is the first demonstration of a solid state ring laser gyroscope or RLG.

A LiNbO₃ integrated optic circuit (IOC) containing eight optical functions has been successfully incorporated into an interferometric fiber optic gyroscope. The IOC has the minimum configuration' optical functions (a phase modulator, a polarizer, and two beam splitters) and Jet Propulsion Laboratory's novel beat detection circuit2 (a phase modulator, two optical taps, and a beam splitter) which provides a means of directly reading angular position and rotation rate. The optical subsystem consisting of the fiber pigtailed IOC and a sensing coil of 945 meters of AT&T polarization maintaining fiber has a loss of 18.7dB, which includes 9dB due to the architecture and unpolarized source. A random walk coefficient of 7.7 x 10-4 deg/NI-Fir was measured using an edge-emitting LED as the source.

Navigation grade fiber optic rotation sensors (FORS) are being developed as an alternative to spinning mass gyro's for unmanned planetary exploration spacecraft. FORS is attractive because of its many advantages such as long life, low weight, low power and low cost as compared to its mechanical counterparts. FORS incorporates an advanced integrated optics circuits fabricated for JPL by AT&T Bell Laboratories. The advanced 8-component integrated optics circuit performs all the key signal processing functions and in addition incorporates a unique optical beat detection circuit (OBDC)2 thereby providing an output in the form of pulses proportional to incremental angular position similar to a ring laser gyro (RLG) but without the inherent lock-in problem RLG's possess.

We have investigated the amplitude variation effects in a serrodyne fiber-optic gyroscope (FOG). As the results of numerical calculation, it became clear that there are bias, scale factor error and discontinuity in regard to the linear relationship between serrodyne frequency and sagnac phase shift. And we confirmed those errors experimentally.

Multi-function integrated-optic chips (MFIOC's) have been identified as necessary components in achieving compact and light weight, low cost fiber-optic gyroscopes [1-6]. MFIOC's offer the advantage of replacing several of the components comprising a minimum reciprocal configuration gyro [7]. At the same time, they provide a means of implementing the phase modulation required in closed loop configurations. In this way a significant reduction of size over gyros employing all discrete components is realized. Additionally, volume production compatible chip fabrication should provide considerable cost reductions.

An open loop fiber gyro being productized for application in business aviation AHRS is described. Reduction of polarization modulation induced quadrature signal is explained. The bias drift over temperature is measured to be < 3 deg/hr. (10, and the scale factor stability over temperature is measured to be < 200 ppm (1a).

Demonstration of better than 1.0 deg/hr bias stability and 0.05 deg/rt.hr angle random walk have been achieved over temperature with a fiber optic gyro under open loop testing conditions. Based on this performance, 0.1 deg/hr bias and .02 deg/rt.hr random walk are realizable system level specifications for low-cost attitude grade IMUs in production.

Advances in modern warfare require the development of artillery munitions that travel beyond the visual horizon. It is not unusual, particularly in undulating terrain, for the direct line of sight from the forward edge of the battle area (FEBA) to the attacking forces to be limited to 2,000 meters. In addition to terrain, influences of clouds, fog, rain and smoke combine to limit the direct line of sight. Mobile targets also decrease probability of kill. What is called for and what has been developed are "smart munitions" that allow the artilleryman to "fire and forget." The technique which accommodates this fire and forget philosophy utilizes a projectile having a radiation detector (or seeker) to sense the target and inertial rate sensors supported by a computer or processor. However, even though we have smart shells today room for improvement exists in weight, power, shelf life, environmental ruggedness and cost.

A fiber gyro concept different from the standard concept is presented. First me-asurements at constant temperature show a bias uncertainty <8°/h and a scale factor accuracy < 0.1%. Due to the inherent properties of the 3 x 3 coupler used, this concept offers the potential for low cost fiberoptic gyros with low to medium performance.

A novel fiber resonator architecture is presented, employing a 90 deg rotation of the polarization within the birefringent fiber ring. In principle, this concept provides a temperature-independent separation of the resonance dips corresponding to the two resonant polarization states in the ring. The technique thus avoids gross thermally driven errors encountered in the use of ordinary polarization-maintaining rings for resonator fiber-optic gyros. Data from an experimental polarization-rotating ring at 1.3 μm wavelength demonstrates greatly improved thermal stability in the presence of typical imperfections of a practical device.

In development of the fiber optics resonant ring gyro it is important to measure the fiber ring linewidth to high accuracy. This short paper introduces a fast and accurate method for measuring of the resonator linewidth and its free spectral range. Using this technique polarization preserving fiber resonator with finesse of 1045 was measured.

This paper contains an analysis of a fiber-optic ring-resonator made with polarization maintaining fiber. The model, although minimal, incorporates the essential ingredients for characterizing polarization crosstalk in a PM resonant fiber optic gyro.

A theoretical model is developed of ring resonators made from polarization preserving and polarizing fiber. The model predicts significant output variation with temperature, as a result of small amounts of polarization crosstalk in the coupler. It also allows determination of the polarizing fiber attenuation necessary for eliminating the output variation. Finally, a resonator configuration made from polarization preserving fiber is proposed, that shows greatly improved temperature stability without the use of polarizers.

A high performance integrated optical circuit for a broad range of fiber optic gyro applications at 1.3 μm is reported. The chip integrates four directional couplers and six phase modulators to perform fiber loop coupling, bias and serrodyne phase modulation, and on chip interferometric serrodyne phase detection. The chip was fabricated by the annealed-proton-exchange method in LiNbO3 yielding a strongly polarized guided circuit with a polarization extinction ratio of at least 60 dB. The halfwave voltages of the phase modulators were 3.5 V and 5.5 V, the 50% splitting and 10% tapping of directional couplers were well within the design specifications.

An integrated optic based interferometric fiber optic gyro configuration is presented which offers potential for low cost and tactical grade accuracy. The gyro utilizes closed loop signal processing. Bias and scale factor stability data are presented.

This paper presents some analysis of scale factor errors due to imperfect serrodyne waveforms in the Fiber Optic Gyro (FOG). In particular, errors in peak phase due to wavelength, integrated optics, or electronics parameter variations are presented in terms of their effect on percent scale factor error.

A practical closed-loop approach to signal processing in the fiber gyroscope is presented. A triangle wave phase modulation is used in which the slope of the increasing and decreasing sections of the modulation are adjusted independently to compensate for the phase difference due to rotation rate. Accordingly, the phase difference due to rotation is determined solely by the difference in the times spent on each of the legs of the triangle wave. By synchronizing these times to a clock signal, the output becomes a digital measure of rotation angle.

A fiber optic position sensor suitable for use in hazardous environments is described. The sensing element is a diffraction grating having a linear variation in period over its length. When illuminated with a broadband optical source at a fixed angle of incidence, linear translation of the grating causes a linear shift in the wavelength of the detected signal collected at a fixed angle. Maximum error of 0.1% over a range of 25 mm has been demonstrated. The sensor is compared to traditional linear position sensors.

Future aircraft will use fly-by-light control systems with fiber-linked optical sensors for such measurands as temperature, pressure, and linear and angular position. We describe a digital optical sensor which was developed to transmit the angular position of such slowly rotating parts as a throttle or fuel flow control valve on an aircraft. The sensor employs a reflective code plate with ten channels providing a resolution of 0.35 degrees. Two light-emitting diodes with overlapping spectra are used as light sources. A single micro-optic multiplexer-demultiplexer composed of a GRIN rod lens and a miniature grating is used to disperse the spectrum and recombine the spectral components from each channel after reflection by the code plate. We discuss the results of preliminary environmental tests of this unit. We have operated the sensor for brief periods of time between -60°C and +125°C without adverse effects. Preliminary vibration tests indicate that the unit will work properly at the maximum vibration levels expected in a jet-engine environment. Use of a reference track on the code plate in conjunction with an automatic gain control in the electronic circuit compensates for large changes in transmitted light intensity.

A fiber optic linear displacement sensor has been developed in which a pair of matched interference filters are used to encode linear position on a broadband optical signal as relative intensity variations. As the filters are displaced, the optical beam illuminates varying amounts of each filter. Determination of the relative intensities at each filter pairs' passband is based on measurements acquired with matching filters and photodetectors. Source power variation induced errors are minimized by basing determination of linear position on signal Visibility. A theoretical prediction of the sensor's performance is developed and compared with experiments performed in the near IR spectral region using large core multimode optical fiber.

A model of intensity based fiber optic displacement sensors is developed. Two sensor designs are presented. The first was designed to measure the gap between a shaft and housing, for use in a magnetic bearing control loop. It is sensitive and has low temperature drift. Another sensor incorporating a scheme to compensate for changes in target reflectivity was also built.

A new class of contactless optical position sensors is presented. These sensors are low cost, easy to fabricate and have wide applications. The sensing mechanism consists of injecting a light beam into a lossy waveguide at the position to be sensed. The coupled light divides equally into two guided waves that attenuate as they propagate in opposing directions towards the extremities of the guide. The ratio of the detected signals is a monotonic function of the position to be sensed. Two prototype sensors were tested. The first sensor consists of a fluorescent strip where external light is actively captured by absorption and reemission at the fluorescence wavelength. The second sensor is a surface-roughened polymer waveguide where light is passively coupled by scattering at the surface irregularities. Both sensors exhibited linear responses over their respective ranges of 10 and 12 centimeters.

Allison Gas Turbine Division of General Motors Corporation, with Teledyne Ryan Electronics, has developed and demonstrated a fiber optic speed sensor (FOSS) for application to advanced fiber optic based control systems for gas turbine engines. The subject FOSS is capable of measuring turbine engine rotational speed throughout a speed range of 0-22,000+ rpm, which is equivalent to a sensor bandwidth of approximately 25 KHz. The principle of operation is based on directly sensing the pressure pulses generated by passing turbine blades via a pressure tube, a diaphragm at the top of the pressure tube, and a fiber optic microbend transducer affixed to the diaphragm. Transceiver electronics/optoelectronics are remotely located.

A temperature sensor is described which consists of a silicon etalon that is sputtered directly onto the end of an optical fiber. A two-layer protective cap structure is used to improve the sensor's long-term stability. The sensor's output is wavelength encoded to provide a high degree of immunity from cable and connector effects. This sensor is extremely compact and potentially inexpensive.

We have demonstrated the performance of a fiber-optic intensity sensor for temperature measurement. The method measures the change in absorption with the temperature of light reflected from a thin silicon film deposited on the end of an optical fiber. Our single-ended sensing approach requires that the incident light intensity be reflected from both film surfaces and that these reflected signals re-enter the fiber. Simplifications in sensor fabrication have been shown to yield sensitivity comparable to that reported by other authors. Calibration data on the sensor is presented up to 400°C. Potential applications of the method are discussed.

Variations in ambient temperature have a negative effect on the performance of any fiber optic sensing system. A change in ambient temperature may alter the design parameters of fiber optic cables, connectors, sources, detectors, and other fiber optic components and eventually the performance of the entire system. The thermal stability of components is especially important in a system which employs intensity modulated sensors. Several referencing schemes have been developed to account for the variable losses that occur within the system. However, none of these conventional compensating techniques can be used to stabilize the thermal drift of the light source in a system based on the spectral properties of the sensor material. The compensation for changes in ambient temperature becomes especially important in fiber optic thermometers doped with rare earths. Thus, the purpose of this work is to search and analyze different approaches to solving this problem.

A double-polarization interferometer with digital readout by fringe counting allows for discrimination between strain applied to the 'reference'- and 'signal'-arm respectively under transverse force induced fiber tension bending. This is utilized for laterally resolved displacement sensing by employing an array of interferometer arms placed under an elastic skin. Experimental results of the one-dimensional field of strain-displacement characteristics employing the four parallel arms of two Michelson interferometers are compared with the theoretically expected, to first order quadratic dependence of strain on displacement under transverse force.

A great deal of effort has been expended in recent years in developing fiber optic instruments, such as temperature, pressure, mechanical, and chemical sensors. However, very little has been done in the area of ionizing radiation detection using fiber optic based instruments. This is in spite of the inability of conventional radiation detectors to perform dose measurements remote from high field environs or in small volumes, such as in situ or in vivo dosimetery. A microminiature radiation detector coupled to an optical fiber could clearly overcome these conventional detector limitations. The present paper describes such a radiation dosimeter using a storage phosphor coupled to an optical fiber suitable for ionizing radiation dosimetry.

In many aerospace applications there are often size constraints on fiber optic components and systems. Similarly, in some fiber optic subscriber loop applications miniature components and subassemblies save valuable space inside enclosures. In this paper details of the fabrication and performance of microfused couplers, wavelength division multiplexers and interferometers will be given.

A new fiber-optic surface roughness sensor based on polarization changes in light scattered from the measured rough surface is proposed. Polarization-maintaining optical fibers have been used for guiding light to and from the surface and a special fiber-optic head has been designed. The data obtained from the polarization measurements have been correlated with some parameters.

Gas flow rates can be measured with a hot-wire anemometer which employs a fine wire mounted transversely to the gas flow. The wire is heated by an electrical current and the temperature rise, which depends inversely on the flow rate, is determined by the resistance change of the wire. In the optical fiber anemometer the fine wire is replaced by a short segment of gold-coated single-mode optical fiber which forms one arm of an all-fiber Mach-Zehnder interferometer. The gold coating is 0.1 pm thick and covers a 1 cm length of the unjacketed silica fiber; this coated section is mounted transversely within a 1 cm diameter tube through which nitrogen gas flows. The fiber is heated by applying a 5 sec voltage pulse to the gold coating. The increase in the temperature results in an increase in both the length and refractive index of the fiber core, and a corresponding increase in the phase of the light propagating through the sensing arm of the interferometer. The magnitude of this phase change, which is determined by fringe counting, is used to determine the temperature change and hence the flow rate. Convective heat transfer coefficients were determined for the low flow rates (0-2 m/sec) employed in these experiments. The experimental results are in excellent agreement with previous experimental results on low-velocity forced convection. In general the device can measure gas flows with greater precision than a conventional hot-wire device.

The microsensor for measuring temperature with high sensitivity presented here contains liquid crystalline layers obtained by the dispersion of cholesteric liquid crystals in a polymer matrix and placed in the end of the fiber-optic head. Due to its double-source construction and the advanced technology employed in liquid crystal sample preparation, the sensor is able to measure temperature at low range with the high sensitivity up to 0.01 K, simultaneously overcoming persistent problems of classical cholesteric liquid crystals such as chemical and photochemical instabilities which caused significant drift.

The optical fiber humidity sensor is based on the principle of the absorption spectrum of unclad optical fiber coated with colorimetric reagent film. The absorption spectrum changes with relative humidity. The output characteristic and sensitivity of the sensor are discussed. Theoretical results show that the sensitivity of the sensor is related to the diameter and the length of the unclad optical fiber, the thickness and the absorption coefficient of the colorimetric reagent film. A new scheme of the optical fiber humidity sensing is put forward. Tests on experimental models have shown performance coinciding with theory, the measurement upper limit of relative humidity to 96%RH, the sensitivity of 0.0196dB/RH to 0.0342dB/RH and the response time of 2.5 sec.. The sensor is more compact and cheaper.

A new clad probing technique is used to measure the size, number, refractive index and viscosity of liquid droplets sprayed from a pressure nozzle on an uncoated core-clad fiber. The probe monitors the clad mode power loss within the leaky ray zone represented as a three region fiber. Liquid droplets measured are Glycerine, commercial grade Turpentine, Linseed oil and some oil mixtures. The measurement sensitivity depends on probing conditions and clad diameter which is observed experimentally and verified analytically. A maximum sensitivity is obtained for the tapered probe-fiber diameter made equal to the clad thickness. A slowly tapered probe-fiber and a small end angle as well as separation of the sensor-fiber and the probe-fiber further improve the sensitivity. Under the best probing condition for 90-percent Glycerine droplets of - 50 micron diameter and a 50/125 micron sensor fiber with clad refractive index of 1.465 and 0.2 NA, the measured sensitivity per drop is 0.015 and 0.006 dB, respectively, for (10-20) and (100-200) droplets. Sensitivities for different systems are shown. The sensitivity is optimized by choosing proper fiber for known liquids.